Colorectal cancer (CRC) develops as a result of complex interactions between the host mucosal immune response, microbiota, diet components, colon cancer stem cells (CSC) and inflammatory mediators. Rice bran is a `wasted' agricultural byproduct of rice processing that has shown CRC chemopreventive efficacy in numerous animal studies. Our metabolomics investigations revealed novel probiotic metabolism of rice bran using an R03 award, and our completed human studies from a recent R21 award provide compelling support that rice bran is a promising, novel dietary agent to be metabolized by the gut microbiota in favor of reducing CRC. Rice bran is a unique source of plant lipids and contains distinct contents from wheat, oat and other cereal grains. Rice bran is distinguished by gamma oryzanol, a distinct ratio of tocotrienols and tocopherol isoforms, and essential nutrients. Notably, rice bran phenolics, phytic acid, and tricin possess CRC chemoprevention activity. Little, however, is known regarding the microbial byproducts of rice bran that our preliminary studies show may be increasing gut mucosal immunity, decreasing colonic inflammation, and reducing neoplastic growth. The major objective of this proposal is to fill a significant gap in our knowledge regarding the microbiome-mediated mechanisms of action for rice bran associated CRC chemopreventive efficacy. Pre-clinical mice studies and stool results from our pilot human clinical rice bran trial showed that dietary rice bran increases healthy native gut microflora and production of stool metabolites with anti-inflammatory activities, favoring reduced colon tumorigenesis. Human stool extracts collected post- rice bran consumption also showed stronger inhibitory effects on CRC cell viability compared to pre-rice bran consumption samples. These data provide compelling rationale for studying the role of these human microbial populations and the utilization of rice bran by these microbes for CRC growth inhibition. The feasibility of carcinogen-treated humanized mice is now established for CRC, and this approach merits testing of stool samples from CRC survivors before and after daily rice bran consumption. We hypothesize that the phytochemicals in rice bran serve as gut microbial substrates and are metabolized into bioactive microbial metabolites that impact the colonic microenvironment for enhanced efficacy against CRC. Our specific aims are: I) To study the influence of dietary rice bran on the human gut microbiota and the gut microbial metabolism of rice bran for effective CRC chemoprevention.; II) To examine and establish the role of dietary rice bran - human gut microbiota interactions on inflammatory/immune milieu and cancer stem cells in the colonic tumor microenvironment; and III) To identify and define the microbial metabolites of rice bran, and relate their specific metabolic/ molecular signatures with anti-CRC efficacy. The results from these studies will provide rice bran-mediated mechanisms associated with anti-CRC efficacy and promote inclusion of rice bran as a sustainable, global functional food for colon cancer chemoprevention.